Sol-gel based heating element

ABSTRACT

Disclosed is a heating element ( 1 ) comprising an electrically insulating layer ( 3 ) and an electrically conductive layer ( 4 ). At least the electrically conductive layer ( 4 ) is based on a hybrid sol-gel precursor comprising an organosilane compound. Also disclosed is an electrical domestic appliance comprising the above heating element. Examples of such domestic appliance include (steam) irons, hair dryers, hair stylers, steamers and steam cleaners, garment cleaners, heated ironing boards, facial steamers, kettles, pressurized boilers for system irons and cleaners, coffee makers, deep fat fryers, rice cookers, sterilizers, hot plates, hot-pots, grills, space heaters, waffle irons, toasters, ovens, and water flow heaters.

The present invention relates to a heating element comprising anelectrically insulating layer and an electrically conductive layer, aswell as to an electrical domestic appliance comprising such a heatingelement.

In general a (flat) heater system comprises two functional layersapplied on a substrate, namely an electrically insulating layer and anelectrically conductive layer. The electrically conductive layer in theabove-mentioned heating element generally comprises a layer with a highohmic resistance, the resistive layer, as well as a layer with a lowerohmic resistance, which acts as a contacting layer. Heat is generated bypassing an electrical current through the resistive layer. The functionof the insulating layer is to isolate the heat-generating resistiveelement from the substrate, which may be directly accessible from theoutside.

The invention specifically relates to a flat heating element that issuitable for high power densities, for instance for application inlaundry irons and other domestic appliances.

Thick-film processing for making flat heating elements involves curingsteps, which should be limited to a temperature compatible with thesubstrate. For aluminum substrates, the maximum curing temperature israther low, and therefore flat-heating materials based on glasses aregenerally not suitable. Low melting point glasses often contain lead orother undesired metals, which are to be avoided, and they have asignificantly lower thermal expansion coefficient than aluminum andaluminum alloys. Polymer-based materials, such as epoxies, or siliconeresins do not have a sufficient temperature stability for them to beused in a heating element. An important factor in this respect is thetemperature drop across the insulating layer, which can be quitesubstantial and which depends on the thickness of the electricallyinsulating layer. This makes polymer-based materials especiallyunsuitable for high power densities, where the track temperatures caneasily be about 100° C. higher than the heating face of the substratefor an insulating layer that is only 50 μm thick.

WO 02/072495 discloses a composition for application to a substrate toform an electrically conductive coating thereon. The compositionincludes a sol-gel solution filled with conductive powder. The sol-gelsolution comprises a non-hybrid sol-gel, such as alumina sol-gel orsilica sol-gel. WO02/072495 also discloses a heating device comprisingthe above composition, in which a thick insulating layer of up to about500 μm is applied. In order to protect the conductive layer againstoxidation, an oxidation barrier layer has to be applied over saidconductive heating layer. This treatment makes the device less sensitiveto corrosion but introduces extra processing steps.

The present invention aims to provide a heating element according to thepreamble which does not have the above disadvantages and which providesa relatively high power density. Moreover, the present invention aims toprovide a heating element that can advantageously be applied on aluminumand aluminum alloy substrates.

To this end the present invention provides a heating element comprisingan electrically insulating layer and an electrically conductive layer,wherein at least the electrically conductive layer is based on a hybridsol-gel precursor comprising an organosilane compound.

By applying such a hybrid sol-gel precursor, a heating element can beprovided with higher power densities and a reduced risk of oxidation ofthe conductive layer. The hybrid sol-gel precursors as disclosed in thepresent invention differ from the non-hybrid precursors as disclosed inWO 02/072495. The hybrid sol-gel precursors as used herein can becharacterized as compounds comprising silicon, which is bound to atleast one non-hydrolyzable organic group, and 2 or 3 hydrolyzable alkoxygroups. The application of the hybrid sol-gel precursors according tothe present invention results in a heating element with veryadvantageous properties.

According to the present invention, at least the electrically conductivelayer is based on a hybrid sol-gel precursor. Advantageously, also theelectrically insulating layer is based on a hybrid sol-gel precursor.Such an electrically insulating layer is also disclosed in WO 02/085072.

The sol-gel material according to the present invention can be processedat temperatures below 450°, which makes them suitable to be applieddirectly to aluminum substrates. Although the sol-gel material isespecially suitable for application on aluminum or aluminum alloysubstrates, other substrates that are conventionally used for heatingelements and that are compatible with the final utility may also beused. Said substrates may comprise, for example, stainless steel,enameled steel, or copper. The substrate may be in the form of a flatplate, a tube, or any other configuration that is compatible with thefinal utility.

In particular, the hybrid sol-gel precursor comprises an organosilanecompound from the group of alkyl-alkoxysilanes.

Preferably, the hybrid sol-gel precursor comprisesmethyl-trimethoxysilane and/or methyl-triethoxysilane.

The hybrid sol-gel precursors according to the present invention shouldbe used in order to obtain a heating element with a relatively highpower density, a reduced risk of oxidation of the resistive layer, andoptimized thermal expansion coefficient values for aluminum and aluminumalloys. Hybrid sol-gel precursors such as methyl-trimethoxysilane (MTMS)and methyl-triethoxysilane (MTES) are known to have an excellenttemperature stability up to at least 450° C. Moreover, MTMS has beenshown to prevent silver oxidation and subsequent migration effectively.The carbon content of these materials is still low, so carbonizedconductive tracks across the insulating layer will not form afterfailure, resulting in a safe flat heating element. The maximum layerthickness of coatings made from hybrid precursors is relatively high,compared with the maximum layer thickness of coatings made fromnon-hybrid sol gel materials. Therefore the layers can be deposited inone or at most two steps without intermediate curing.

Advantageously, the electrically insulating layer comprisesnon-conductive particles.

A fraction of said non-conductive particles preferably have a flake-likeshape and a longest dimension of 2-500 micrometers, preferably from2-150 micrometers, and more preferably from 5-60 micrometers. Theseflake-like non-conductive particles are based on oxidic materials suchas, for example, mica, or clay, and/or surface-modified mica or clayparticles with a coating of titanium dioxide, aluminum oxide and/orsilicon dioxide. The flake-like material content in the insulating layershould be less than 20%, preferably less than 15%, and more preferably4-10% by volume.

An advantage of such anisotropic particles is that their presenceprevents the formation of cracks in the electrically insulating layerafter frequent heating up and cooling down of the heating element.

In the preferred embodiment, the other non-conductive particles arepresent in a colloidal form. Examples thereof are oxidic materials likealuminum oxide and silicon dioxide. Preferably, the aluminum oxidecontent in the insulating layer should be less than 40%, preferably lessthan 20%, more preferably 10-15% by volume. As for the silicon dioxidecontent in the insulating layer, it should advantageously be less than50%, preferably less than 35%, more preferably 15-25% by volume.

If an insulating layer based on MTMS or MTES filled with particles,including anisotropic particles, is made, a layer thickness of just 50μm can withstand 5000V. This relatively small layer thickness allows thetemperature of the resistive track to be fairly low. For a specific highpower density application of 50 W/cm² that requires a heating facetemperature of 250° C., a conductive track temperature of only 320° C.is required. By contrast, an excess temperature of the heat-generatingconductive layer of about 600° C. would be required for an insulatinglayer thickness of 300 μm. For this reason said thin insulating layersare advantageously used. The layers can be applied by any wet chemicalapplication method, preferably spray coating or screen printing followedby a curing step.

The heating elements according to the present invention are verysuitable for use as heating elements in laundry irons, especially forthe controlled formation of steam, for which high power densities arerequired. However, the heating elements are also very suitable for otherdomestic appliances, like hair dryers, hair stylers, steamers and steamcleaners, garment cleaners, heated ironing boards, facial steamers,kettles, pressurized boilers for system irons and cleaners, coffeemakers, deep-fat fryers, rice cookers, sterilizers, hot plates,hot-pots, grills, space heaters, waffle irons, toasters, ovens, or waterflow heaters.

The heating element according to the present invention as well as theprocessing steps for providing said heating element will be described inmore detail below by way of example.

The materials and processes are designed for applying a thin heatingelement to a metal substrate such as aluminum. A hybrid sol-gelsolution, made preferably of MTMS or MTES, water and filled with oxidicparticles such as silica, alumina and titania is prepared for theinsulating layer by hydrolysis using a suitable acid. It was found to bespecifically beneficial to include strongly anisotropic particles, suchas mica or commercially available interference pigments, in the formulato maintain high dielectric breakdown strengths during use. This coatingliquid can be applied to an aluminum substrate, preferably an anodizedaluminum substrate, to ensure good adhesion of the sol-gel layer.Normally two layers are sprayed, with a short intermediate drying step,but without the need of an intermediate curing step. This leads to afinal coating layer thickness of about 50 μm. Advantageously, theinsulation layer has a thickness of 25-100 μm, preferably 35-80 μm.

Curing takes place at a temperature of around 415° C., depending on thesubstrate and application requirements.

A conductive layer or track is applied on top of the insulating layer.Advantageously, the electrically conductive layer comprises conductiveand/or semi-conductive particles, as well as an amount of insulatingparticles of 0-20% by volume. The insulating particles may be added tomodify the resistance of the layer or track.

Advantageously, the electrically conductive layer does not exceed 30 μmin thickness and preferably does not exceed 15 μm in thickness.

The preferred technique for applying the conductive tracks is screenprinting. Commercially available metal powders can be used for theconductive track. It is preferred to use silver or silver alloyparticles. Mixing of the silver particles with palladium particles orthe use of silver-palladium alloys both lead to a change in resistivitywhile the positive temperature coefficient value is reduced. Graphitemay also be used to advantage to make conductive tracks. Other metalsand semiconductors may be used in making conductive layers for theapplication, provided they have a sufficiently high temperaturestability in the hybrid sol-gel matrix. The use of MTMS or MTESprecursors reduces the rate of oxidation of silver and graphiteparticles at the high temperatures in application. In that respect itcan be noted that graphite in an MTES-derived matrix has shown along-term stability (over 600 hours) at 320° C.

The conductivity achieved depends on the volume fraction of conductiveparticles in the conductive layer, and can be further influenced by theaddition of non-conductive particles. The addition of non-conductiveparticles may either increase or decrease the layer conductivity.

To make the formula screen-printable, a cellulose derivative is added tothe particle-containing, hydrolyzed MTMS or MTES solution.Hydroxy-propyl-methyl-cellulose is preferably used as the cellulosematerial. Finally a solvent with a high boiling point is added toprevent drying of the ink and subsequent clogging of the screen.Butoxyethanol was found to be a suitable choice, but other polarsolvents, preferably alcohols, are also appropriate.

A protective layer to prevent corrosion is not needed on this stack oflayers. However, for the sake of mechanical integrity during handlingand production it may be beneficial to deposit such a layer. Using forinstance a silica-filled hybrid sol-gel solution based on, for example,MTMS, a screen printable formula can be easily made. The applied topcoatlayer can be co-cured with the conductive layers.

The heating elements thus prepared were subjected to over 600temperature cycles in which the element was maintained at 320° C. for 1hour, and subsequently switched off during 30 minutes. The hightemperature was obtained bypassing an electrical current through theconductive layers, by which power densities of 10 to at least 120 W/cm²could be achieved.

The invention will be further elucidated with reference to the followingembodiment, the following manufacturing examples, and the encloseddrawing, in which:

FIG. 1 is a sectional view of an embodiment of the heating elementaccording to the present invention.

It is noted that the various elements are purely schematic and are notdrawn to scale.

The heating element 1 as shown in FIG. 1 is built up of a substrate 2,an insulating layer 3, and an electrically conductive layer or resistivelayer 4.

In the embodiment shown, the substrate 2 comprises aluminum or analuminum alloy which is used for a sole plate of an iron. Said substrate2 is covered with a layer 3 of an electrically insulating material. Inthe example, the electrically insulating layer 3 is based on a hybridsol-gel precursor and has a thickness of 50 μm. The resistive layer 4comprises a track of a conductive coating—not specifically shown in theFigure—with a high ohmic resistance, which is, in the present example,screen-printed on the insulating layer 3.

EXAMPLE 1

A lacquer was prepared from 32.82 g of MethylTriMethoxySilane (MTMS)12.62 g aluminum oxide CR6 (Baikalox), 16.41 g ethanol, 0.31 g maleicacid, and 34.95 g of a colloidal silica suspension Bindzil 40NH3/80 (EKAChemicals). The water from the silica suspension was used to hydrolyzethe alkoxysilanes. 2.89 g of a commercially available flake-like, micabased pigment was added to the lacquer to reduce the sensitivity tocrack formation.

After completion of the hydrolysis reaction, the lacquer wasspray-coated onto a 3 mm thick, anodized aluminum substrate. The anodiclayer thickness was less than 4 microns and served as a primer layer forthe sol-gel insulating layer.

The layers were subsequently cured at 415° C. to obtain a dry filmthickness of 50 μm. The dielectric strength of this layer is higher than10⁸ volt/m. This coating was able to withstand more than 1000 cycles ofheating-up to 320° C. and cooling-down to room temperature. After 1000cycles, still no crack formation was observed and no deterioration ofthe dielectric breakdown strength was measured.

COMPARATIVE EXAMPLE 1

A coating similar to that in example 1 was prepared, except for theaddition of the flake-like pigment to the lacquer. The dielectricstrength of this layer is higher than 10⁸ V/m. This coating was able towithstand only 300 cycles of heating-up to 320° C. and cooling-down toroom temperature. After 300 cycles a severe crack formation wasobserved, leading to breakdown voltages of less than 600 V, which is toolow for application in domestic appliances.

EXAMPLE 2

A heating element was prepared starting from an aluminum substrateprovided with an insulating layer as described in example 1. Onto thislayer a conductive track was printed using a paste prepared according tothe recipe given below.

A hydrolysis mixture was prepared from 84.8 g methyltriethoxysilane,51.2 g water, and 0.24 g glacial acetic acid. The mixture was stirredcontinuously for 5 hours. 3.85 g Disperbyk 190 was added to 36 g of thishydrolysis mixture followed by 77.8 g of a commercially available silverpowder with a particle size below 20 μm. Subsequently 36 g n-propanolwas added to the mixture, which was subsequently ball-milled overnighton a roller conveyor.

After removal of the milling balls, 35 g of a 6%hydroxypropyl-methylcellulose solution in water was added to 120 g ofthe mixture. After mixing a homogeneous paste was obtained which wasscreen-printed on said insulating sol-gel layer. The layers were driedat 80° C. and subsequently cured at 415° C. A single layer had athickness of about 5 μm and a sheet resistance of 0.046 Ω per square.The quality was such that the sample-to-sample variation of the sheetresistance was less than 5%. The heating element was powered up activelyby application of an electrical current through said conductive layer toobtain a temperature of 320° C. The sheet resistance was found todecrease to a plateau value of about 20% below the initial resistancevalue after prolonged exposure to said temperature. This plateau valuewas reached within 60 hours of exposure to said temperature.

EXAMPLE 3

A similar heating element as described in example 2 was prepared, exceptfor the fact that the conductive layer printing was repeated afterdrying of the first printed conductive layer. After drying and curing ofthe conductive layer stack, a layer thickness of 10 μm was measured. Thedouble-pass printed conductive layers had a sheet resistance of 0.024 Ωper square. The quality was such that the sample-to-sample variation ofthe sheet resistance was less than 5%. The heating element was poweredup actively by application of an electrical current through saidconductive layer to obtain a temperature of 320° C. The sheet resistancewas found to decrease to a plateau value of about 20% below the initialresistance value after prolonged exposure to said temperature. Thisplateau value was reached within 60 hours of exposure to saidtemperature.

EXAMPLE 4

A heating element was prepared starting from an aluminum substrateprovided with an insulating layer as described in example 1. Conductiveand contacting tracks were printed onto this layer, using pastematerials prepared according to the recipes given below.

A hydrolysis mixture was prepared from 56.0 g methyltriethoxysilane,33.8 g water, and 0.16 g glacial acetic acid. The mixture was stirredcontinuously for 5 hours, after which 7.95 g of Disperbyk 190 was addedfollowed by 31.74 g graphite powder with a particle size of around 10μm.

The mixture was ball-milled overnight on a roller conveyor. Afterremoval of the milling balls, 60 g a 6% hydroxypropyl-methylcellulosesolution in water was added to 100 g of the mixture, followed by 50 gn-propanol. After mixing, a homogeneous paste was obtained which wasscreen-printed on said insulating sol-gel layer to form a conductivelayer.

After drying of the conductive layer, a contacting layer based on therecipe disclosed in example 2 was screen-printed on said substrate. Thecontacting layer partly overlapped the conductive layer to form alow-ohmic contact.

The screen-printed layers were dried at 80° C. and subsequently cured at415° C. A layer thickness of about 5 μm was obtained with single passprinting. The sheet resistance of the conductive layer was 57 Ω persquare. The quality was such that the sample-to-sample variation of thesheet resistance was less than 10%. The heating element was powered upactively by application of an electrical current through said conductivelayers to obtain a temperature of 320° C. Prolonged exposure to saidtemperatures did not show any significant change in sheet resistance.

EXAMPLE 5

A similar heating element as described in example 4 was prepared, exceptfor the fact that the contacting layer was applied before the conductivelayer. The sheet resistance of the conductive layer was 57 Ω per square.The quality was such that the sample-to-sample variation of the sheetresistance was less than 10%. The heating element was powered upactively by application of an electrical current through said conductivelayers to obtain a temperature of 320° C. Prolonged exposure to saidtemperatures did not show any significant change in sheet resistance.

EXAMPLE 6

A similar heating element as described in example 4 was prepared, exceptfor the fact that the conductive layer printing was repeated afterdrying of the first printed conductive layer. After drying and curing ofthe conductive layer stack, a layer thickness of 10 μm was measured. Thedouble-pass printed conductive layers had a sheet resistance of 26 Ω persquare. The quality was such that the sample-to-sample variation of thesheet resistance was less than 10%. The heating element was powered upactively by application of an electrical current through said conductivelayers to obtain a temperature of 320° C. Prolonged exposure to saidtemperatures did not show any significant change in sheet resistance.

EXAMPLE 7

A heating element was prepared starting from an aluminum substrateprovided with an insulating layer as described in example 1. Aconductive track was printed onto this layer, using a paste preparedaccording to the recipe given below.

To 16 g the hydrolysis mixture as described in example 4, 1.7 gDisperbyk 190 was added, followed by 35 g a commercially availablesilver powder (particle diameter smaller than 20 μm), 1.35 g Al₂O₃(Baikalox CR6), and 16 g 1-propanol. This mixture was ball-milledovernight. After removal of the milling balls, 13 g a 6% HPMC solutionin water was added, and the resulting paste was screen-printed onto saidinsulating sol-gel layer. After drying at 80° C. and curing at 415° C.,a layer thickness of 6 μm resulted with a sheet resistance of 0.07 Ω persquare.

EXAMPLE 8

A flat heating element was prepared according the description in example2, with the difference that a hybrid topcoat layer was printed afterprinting of the conductive layer.

The topcoat was prepared from a hydrolysis mixture based on 37.35 gmethyltriethoxysilane, 22.55 g water, and 0.10 g glacial acetic acid.The mixture was stirred continuously for 5 hours, after which 9.6 gDisperbyk 190 was added, followed by 41.0 g titanium dioxide powder witha particle size of around 250 nm. The mixture was ball-milled overnighton a roller conveyor. After removal of the milling balls, 36 g of a 6%hydroxypropyl-methylcellulose solution in water was added to 60 g of thesuspension, followed by 30 g n-propanol. After mixing, a homogeneouspaste was obtained which was screen-printed over the complete flatheating element except for two contacting pads. The topcoatscreen-printing step was carried out before said curing treatment of theconductive layer. After drying at 80° C., the complete coating stack wascured at a temperature of 350° C.

The measured resistance of the conductive track was 0.047 Ω per square.

EXAMPLE 9

A flat heating element was prepared according to the description inexample 2, with the difference that a defect in the conductive layer wasinduced by means of the placement of a human hair on the substratebefore the application of the conductive layer. After the conductivelayer had been printed, the human hair was removed, leaving behind adefect in the conductive layer.

The heating element was subsequently dried at 80° C. followed by acuring step at 350° C.

Next the heating element was powered up by application of a current of 9A induced by an alternating voltage difference of 220 V. Sparking of theelement was observed at the position of the hair defect in theconductive layer, leading to failure of the element. The quality of theinsulating layer was tested by application of a potential difference of1250 V between the conductive track and the aluminum substrate for aperiod of 60 seconds. The leakage current was measured to be less than 1mA, fulfilling the safety requirements.

1. A heating element comprising an electrically insulating layer and anelectrically conductive layer, wherein at least the electricallyconductive layer is based on a hybrid sol-gel precursor comprising anorganosilane compound.
 2. A heating element according to claim 1,characterized in that the hybrid sol-gel precursor comprises a compoundfrom the group of alkyl-alkoxysilanes.
 3. A heating element according toclaim 1, characterized in that the hybrid sol-gel precursor comprisesmethyl-trimethoxysilane and/or methyl-triethoxysilane.
 4. A heatingelement according to claim 1, characterized in that the electricallyinsulating layer comprises non-conductive particles.
 5. A heatingelement according to claim 4, characterized in that the electricallyinsulating layer comprises anisotropic, non-conductive particles.
 6. Aheating element according to claim 1, characterized in that theelectrically conductive layer comprises conductive and/orsemi-conductive particles, as well as an amount of insulating particlesin a quantity of 0-20% by volume.
 7. A heating element according toclaim 6, characterized in that the electrically conductive layercomprises metal particles.
 8. A heating element according to claim 7,characterized in that the electrically conductive layer comprises silveror silver alloy particles.
 9. A heating element according to claim 6,characterized in that the electrically conductive layer comprisesgraphite or carbon-black particles.
 10. A heating element according toclaim 1, characterized in that the electrically conductive layer doesnot exceed 30 μm in thickness and preferably does not exceed 15 μm inthickness.
 11. A heating element according to claim 1 comprising aninsulating layer having a thickness of 25-100 μm, preferably 35-80 μm.12. A heating element according to claim 1, applied on an aluminum oraluminum alloy substrate.
 13. An electrical domestic appliancecomprising at least a heating element in accordance with claim
 1. 14. Anelectrical domestic appliance according to claim 13, characterized inthat the electrical domestic appliance comprises a (steam) iron, hairdryer, hair styler, steamer and steam cleaner, garment cleaner, heatedironing board, facial steamer, kettle, pressurized boiler for systemirons and cleaners, coffee maker, deep fat fryer, rice cooker,sterilizer, hot plate, hot-pot, grill, space heater, waffle iron,toaster, oven, or water flow heater.